In general, refrigerators such as compression-type refrigerators comprises a compressor, a condenser, an expansion mechanism like an expansion valve, an evaporator and, where necessary, a dryer and has a structure in which a mixed fluid of a refrigerant and a lubricating oil is circulated in the closed system. In the compression-type refrigerator, in general, temperature is high in the compressor and low in the refrigerating chamber although the conditions may be different depending on the type of the apparatus and it is required, in general, that the refrigerant and the lubricating oil be circulated in the system without causing phase separation in a wide temperature range.
In general, there are two areas of phase separation between a refrigerant and a lubricating oil, i.e., at the lower temperature side and at the high temperature side. It is preferable that the maximum temperature in the area of phase separation at the low temperature side is 10° C. or lower, more preferably 5° C. or lower, still more preferably 3° C. or lower, still more preferably 0° C. or lower and most preferably −2° C. or lower. When the phase separation takes place during the operation of refrigeration, the lifetime and the efficiency of the apparatus are adversely affected to a great extent. For example, when the phase separation of the refrigerant and the lubricating oil takes place in the compressor, lubrication of moving parts deteriorates and seizure occurs to cause a great decrease in the lifetime of the apparatus. When the phase separation takes places in the evaporator, the efficiency of heat exchange decreases because of the presence of lubricating oil of high viscosity.
As the refrigerant for the compression-type refrigerators and, in particular, for air conditioners, heretofore, chlorodifluoromethane (referred to as R22, hereinafter) and a mixture of chlorodifluoromethane and chloropentafluoroethane in weight ratio of 48.8:51.2 (referred to as R502, hereinafter) have been mainly used. As the lubricating oil, various types of mineral oils and synthetic oils that satisfy the foregoing requirement have been used. However, R22 and R502 are more rigorously restricted worldwide because there is the anxiety of causing environmental pollution such as destruction of the ozonosphere present in the stratosphere. As the novel type of the refrigerant, hydrofluorocarbons such as 1,1,1,2-tetrafluoroethane, difluoromethane, pentafluoroethane and 1,1,1-trifluoroethane (occasionally referred to as R134a, R32, R125 and R143a, respectively) are attracting attention. These hydrofluorocarbons and, in particular, R134a, R32, R125 and R143a have little anxiety of causing the ozonosphere destruction and are preferable as the refrigerant for the compression-type refrigerators.
Further improvements are required from the standpoint of the energy saving and difluoromethane (R32) among the foregoing novel refrigerants is attracting attention. However, the refrigerant R32 is used at a higher pressure and a higher temperature than those of conventional refrigerants and there is the great possibility that problems arise on lubrication. It is the actual present situation that no lubricating oils exhibiting sufficient miscibility with R32 are found among the lubricating oils for refrigerants that have been examined for the use in combination with the foregoing novel refrigerants. Development of a lubricating oil exhibiting miscibility with R32 at the same level as that with R134a, R407c (a mixture of R32, R125 and R134a in the weight ratio of 23:24:52) and R410A (a mixture of R32 and R125 in the weight ratio of 50:50) has been desired.
Under the above circumstances, the present invention has an object of providing a lubricating oil for refrigerators which exhibits excellent miscibility with a refrigerant having one carbon atom and no chlorine atoms and, in particular, with difluoromethane (R32). Another object of the present invention is to provide a hydraulic fluid composition for refrigerators comprising the lubricating oil for refrigerators and the refrigerant and exhibiting excellent antiwear, lubricity and stability.
The present inventors have been studying miscibility in the presence of the refrigerant R32 regarding oxygenated compounds such as polyether compounds, examples of which include polyoxyalkylene glycol derivatives and polyvinyl ether derivatives, and polyol ester derivatives. However, these compounds do not always exhibit the sufficient miscibility with the refrigerant R32.